The distances between objects in the night sky are measured as angles. An
object on the horizon is said to lie at a distance of 90° from an object
vertically above the observer, at the zenith.

A crude estimate of the distance between two objects may be found visually, by
holding a fist at arm's length: for a typical adult the fist will measure
around 10° across. The disk of the Moon measures almost exactly half a
degree across.

For smaller distances, for example between the individual
members of the Pleiades star cluster, it is convenient to use a measure of
angle which is smaller than the degree: the arcminute is used, sometimes
denoted ', which equals one sixtieth of a degree. The angular
resolution of an unaided human eye, with near perfect eyesight, is around one
to two arcminutes.

Still smaller angles are measured in arcseconds, sometimes denoted ",
which equal one sixtieth of an arcminute, or one 3,600th of a degree. For
example, the angular diameter of Jupiter varies between 30 and 50"
depending upon its position in the night sky.

The smallest structures which
can be seen through a conventional ground-based telescope, however large, are
around one arcsecond across, as smaller structures are blurred by turbulence
in the Earth's atmosphere. Some modern telescopes, however, can get around
this limitation by the use of adaptive optics systems, or by being launched
into space.